Several publications are referenced in this application in order to more fully describe the state of the art to which this invention pertains. The disclosure of each of these publications is incorporated by reference herein.
The green fluorescent proteins (GFPs) are a unique class of chromoproteins found in many bioluminescent hydrozoan and anthozoan coelenterates, including the hydromedusan jellyfish (Aequorea victoria). The gene for A. victoria GFP has been cloned (Prasher et al., 1992, Gene, 111:229-233) and expression of GFP in prokaryotic and eukaryotic hosts results in the synthesis of a functional fluorescent protein with spectral characteristics identical to that of native A. victoria GFP (Chalfie et al., 1994, Science, 263:802-805)
GFP is a 238 amino acid protein which has an excitation spectrum characterized by a major excitation peak at 395 nm (blue light), a minor excitation peak at 470 nm, and an emission peak at 509 nm (green light). The GFP absorption bands and emission peak arise from an internal p-hydroxybenzylidene-imidazolidinone chromophore, which is generated by cyclization and oxidation of a Ser-Tyr-Gly (SYG) sequence at residues 65-67 (Cody et al., 1993, Biochemistry 32:1212-1218.
Since fluorescence emission by GFP does not require tissue fixation, exogenous substrates, and/or cofactors, it has become the reporter of choice for studies that require detection of exogenously expressed proteins in living cells and organisms. GFP has been used extensively in a variety of studies to monitor gene expression, cell development, or protein localization (i.e., Chalfie et al., 1994, Science 263:802-805; Heim et al., 1994, Proc. Nat. Acad. Sci. 91:12501-12504; Chalfie and Prasher, WO 95/07463, Mar. 16, 1995). Wild-type GFP has also been used as a tool for visualizing subcellular organelles (Rizzuto et al., 1995, Curr. Biology 5:635-642) and protein transport along a secretory pathway (Kaether and Gerdes, 1995, Febs Letters 369:267-271). The expression of GFP in plant cells (Hu and Cheng, 1995, Febs Letters 369:331-334) and Drosophila embryos (Davis et al., 1995, Dev. Biology 170:726-729) has also been described. Such experiments have been performed wherein GFP or a GFP-tagged fusion protein was expressed in a desired cell or cell population, the expression of which was detected by excitation/emission spectra of the exogenous fluorescent protein. It should be noted, however, that GFP and GFP fusion proteins can also be visualized in fixed cells and tissue.
The crystal structures of wild-type GFP and the GFP S65T mutant have been solved and reveal that the GFP tertiary structure resembles a barrel (Ormo et al., 1996, Science 273:1392-1395; Yang, et al., 1996, Nature Biotech 14: 1246-1251). The barrel consists of beta sheets in a compact anti-parallel structure, within which an alpha helix containing the chromophore is contained. As a consequence of this compact structure, GFP is a very stable protein even when exposed to harsh conditions such as protease treatment. The inherent stability of GFP, therefore, renders it an ideal reporter protein in a variety of biological systems. The stability of GFP is, however, problematic in applications requiring detection of rapid or repetitive events.
To expand the utility of GFP to include a broader range of research applications, efforts have been underway to optimize wild-type GFP and identify novel GFP variants to produce GFP reagents. For example, “humanized” GFPs have been generated which are expressed at higher levels in mammalian cells (Haas, et al., 1996, Current Biology 6:315-324; Yang, et al., 1996, Nucl Acids Res 24:4592-4593). Enhanced green fluorescent protein (EGFP) is an example of such a humanized GFP. Mutational screening of GFP DNA sequences has produced mutant GFP DNA sequences which encode GFP variants having different spectral properties, including variants that emit in the blue-, cyan- or yellow-green wavelength.